Field-effect transistor, method for producing the same, display element, image display device, and system

ABSTRACT

A field-effect transistor including: a gate electrode, which is configured to apply gate voltage; a source electrode and a drain electrode, which are configured to take electric current out; an active layer, which is disposed between the source electrode and the drain electrode and is formed of an oxide semiconductor; and a gate insulating layer, which is disposed between the gate electrode and the active layer, the source electrode and the drain electrode each including a metal region formed of a metal and an oxide region formed of one or more metal oxides, and a part of the oxide region in each of the source electrode and the drain electrode being in contact with the active layer, and rest of the oxide region being in contact with one or more components other than the active layer.

TECHNICAL FIELD

The present disclosure relates to a field-effect transistor, a methodfor producing the same, a display element, an image display device, anda system.

BACKGROUND ART

Flat panel displays (FPDs) such as liquid crystal displays (LCDs),organic EL (electroluminescence) displays (OLEDs), and electronic papersare driven by a driving circuit including a thin film transistor (TFT)in which an amorphous silicon or a poly-crystalline silicon is used asan active layer.

In the development of FPDs, there has been attention to produce a TFTincluding a field-effect transistor, which includes, on a channelforming area of the active layer, an oxide semiconductor film havinghigh carrier mobility and small unevenness between the elements, and toapply the TFT to electric devices or optical devices.

In a field-effect transistor including an oxide semiconductor serving asan active layer, a stacked metal film of Al and a barrier layerincluding a transition metal (Mo, Ti, or an alloy thereof), which is atypical metal material, may be used for materials for the sourceelectrode and the drain electrode (for example, see PTL 1). In thatcase, there is concern that the resultant transistor becomes anormally-on field-effect transistor where even when gate voltage of thetransistor is not applied, a large quantity of electric current flowsbetween the source electrode and the drain electrode.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No.2011-216694

SUMMARY OF INVENTION Technical Problem

The present disclosure has an object to provide a field-effecttransistor which prevents an oxide semiconductor to be an active layerfrom being reduced between an interface of the oxide semiconductor witha source electrode and an interface of the oxide semiconductor with adrain electrode, and has a simple configuration and favorable transistorcharacteristics.

Solution to Problem

Means for solving the aforementioned problems are as follows. That is, afield-effect transistor of the present disclosure includes: a gateelectrode, which is configured to apply gate voltage; a source electrodeand a drain electrode, which are configured to take electric currentout; an active layer, which is disposed between the source electrode andthe drain electrode and is formed of an oxide semiconductor; and a gateinsulating layer, which is disposed between the gate electrode and theactive layer. The source electrode and the drain electrode each includea metal region formed of a metal and an oxide region formed of one ormore metal oxides. A part of the oxide region in each of the sourceelectrode and the drain electrode is in contact with the active layer,and the rest of the oxide region is in contact with one or morecomponents other than the active layer.

Advantageous Effects of Invention

According to the present disclosure, it is possible to provide afield-effect transistor which prevents an oxide semiconductor to be anactive layer from being reduced between an interface of the oxidesemiconductor with a source electrode and an interface of the oxidesemiconductor with a drain electrode, and has a simple configuration andfavorable transistor characteristics.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic structural view illustrating one example of afield-effect transistor of the present disclosure.

FIG. 2 is a schematic structural view illustrating another example of afield-effect transistor of the present disclosure.

FIG. 3A is a schematic cross-sectional view illustrating a method forproducing a field-effect transistor of the present disclosure (part 1).

FIG. 3B is a schematic cross-sectional view illustrating a method forproducing a field-effect transistor of the present disclosure (part 2).

FIG. 3C is a schematic cross-sectional view illustrating a method forproducing a field-effect transistor of the present disclosure (part 3).

FIG. 3D is a schematic cross-sectional view illustrating a method forproducing a field-effect transistor of the present disclosure (part 4).

FIG. 3E is a schematic cross-sectional view illustrating a method forproducing a field-effect transistor of the present disclosure (part 5).

FIG. 4 is a schematic structural view illustrating another example of afield-effect transistor of the present disclosure.

FIG. 5 is a schematic structural view illustrating another example of afield-effect transistor of the present disclosure.

FIG. 6 is a schematic structural view illustrating another example of afield-effect transistor of the present disclosure.

FIG. 7 is a schematic structural view illustrating another example of afield-effect transistor of the present disclosure.

FIG. 8 is a schematic structural view illustrating another example of afield-effect transistor of the present disclosure.

FIG. 9 is a schematic structural view illustrating another example of afield-effect transistor of the present disclosure provided as amodification example of FIG. 6.

FIG. 10 is a schematic structural view illustrating another example of afield-effect transistor of the present disclosure provided as amodification example of FIG. 7.

FIG. 11 is a schematic structural view illustrating another example of afield-effect transistor of the present disclosure provided as amodification example of FIG. 8.

FIG. 12 is a diagram for presenting an image display device.

FIG. 13 is a diagram for presenting one example of a display element ofthe present disclosure.

FIG. 14 is a schematic structural view illustrating one example of apositional relationship between an organic EL element and a field-effecttransistor in a display element.

FIG. 15 is a schematic structural view illustrating another example of apositional relationship between an organic EL element and a field-effecttransistor in a display element.

FIG. 16 is a schematic structural view illustrating one example of anorganic EL element.

FIG. 17 is a diagram for presenting a display control device.

FIG. 18 is a diagram for presenting a liquid crystal display.

FIG. 19 is a diagram for presenting a display element in FIG. 18.

FIG. 20 is a schematic structural view illustrating a field-effecttransistor of Comparative Example 1.

FIG. 21 is a schematic structural view illustrating a field-effecttransistor of Comparative Example 2.

DESCRIPTION OF EMBODIMENTS

In the field-effect transistor, it is known that when the oxidesemiconductor serving as the active layer has an electron carrierconcentration of from 1×10¹⁵ cm⁻³ through 1×10¹⁶ cm⁻³ and a metal (forexample, Al) having a low work function is used as the source electrodeand the drain electrode, obtained transistor characteristics are highfield-effect mobility, high on/off ratio and a small absolute value ofthe rising voltage.

Meanwhile, when the electron carrier concentration is 1.0×10¹⁸ cm⁻³ ormore, obtained transistor characteristics are small on/off ratio and alarge absolute value of the rising voltage (for example, see JapaneseUnexamined Patent Application Publication No. 2010-62546).

In addition, it is reported that even when the electron carrierconcentration is 1×10¹⁸ cm⁻³ or more, incorporation of a metal (forexample, Au having a deep work function) into the source electrode andthe drain electrode makes it possible to obtain such transistorcharacteristics as high field-effect mobility, high on/off ratio, and asmall absolute value of the rising voltage (for example, see JapaneseUnexamined Patent Application Publication No. 2015-046568).

As mentioned above, when the electron carrier concentration of the oxidesemiconductor is controlled to a desired value and suitable materialsare selected for the source electrode and the drain electrode, it ispossible to obtain such transistor characteristics as high field-effectmobility, high on/off ratio, and a small absolute value of the risingvoltage.

However, even when the electron carrier concentration of the oxidesemiconductor is controlled to the desired value as described above toform the active layer, unless such a metal that is not oxidized (e.g.,Au or Pt) is used as the source electrode and the drain electrode, thefollowing phenomenon will occur. That is, in a contact interface betweenthe oxide semiconductor and each of the drain electrode and the sourceelectrode, the metal material to be an electrode deprives the oxidesemiconductor of oxygen atoms, and thus oxygen vacancies form in theoxide semiconductor to increase the electron carrier concentration ofthe oxide semiconductor. This may cause unevenness and deterioration ofthe transistor characteristics in the field-effect transistor using theactive layer, the oxide semiconductor that mainly uses the oxygenvacancies as the sources of carrier generation.

For example, in a field-effect transistor using an oxide semiconductorin an active later, a transition metal (e.g., Mo), which is not achemically stable metal unlike Au, Pt, and other chemically stablemetals, is used for materials of the source electrode and the drainelectrode in some cases. In this case, in a region in contact with theoxide semiconductor, the electrode deprives the oxide semiconductor ofoxygen atoms, resulting in reduction of the oxide semiconductor. Thereduction of the oxide semiconductor causes oxygen vacancies of theoxide semiconductor, resulting in an increase in the electron carrierconcentration. As a result, there is a concern that the resultanttransistor becomes a normally-on field-effect transistor where even whengate voltage of the transistor is not applied, a large quantity ofelectric current flows between the source electrode and the drainelectrode.

Under such circumstances, the present inventors conducted extensivestudies. The present inventors have found that it is possible to preventthe oxide semiconductor from being reduced due to a metallic sourceelectrode and a metallic drain electrode by oxidizing the surfaces ofthe source electrode and the drain electrode and oxidizing a part of thesource electrode and a part of the drain electrode, which are in contactwith the active layer. As a result, the present inventors have completedthe present invention.

(Field-Effect Transistor)

A field-effect transistor of the present disclosure includes a gateelectrode, a source electrode, a drain electrode, an active layer, and agate insulating layer, further includes other components if necessary.

<Gate Electrode>

The gate electrode is not particularly limited and may be appropriatelyselected depending on the intended purpose so long as it is an electrodeconfigured to apply gate voltage.

A material of the gate electrode is not particularly limited and may beappropriately selected depending on the intended purpose. Examples ofthe material of the gate electrode include metals (e.g., Al, platinum(Pt), palladium (Pd), gold (Au), silver (Ag), Cu, zinc (Zn), Ni, Cr, Ta,Mo, and Ti), alloys of the metals, and mixtures of these metals. Thematerial of the gate electrode may be conductive oxides (e.g., indiumoxide, zinc oxide, tin oxide, gallium oxide, and niobium oxide),composite compounds thereof, and mixtures thereof.

An average thickness of the gate electrode is not particularly limitedand may be appropriately selected depending on the intended purpose. Theaverage thickness of the gate electrode is preferably from 10 nm through1 micrometer, more preferably from 50 nm through 300 nm.

<Source Electrode and Drain Electrode>

The source electrode and the drain electrode are electrodes configuredto take electric current out.

The source electrode and the drain electrode are formed so that apredetermined interval is disposed between the source electrode and thedrain electrode.

The source electrode and the drain electrode each include a metal regionformed of a metal and an oxide region formed of one or more metaloxides.

A part of the oxide region in each of the source electrode and the drainelectrode is in contact with the active layer, and the rest of the oxideregion is in contact with one or more components other than the activelayer.

When the part of the oxide region is in contact with the active layer,the metal included in the source electrode and the drain electrodeprevents the active layer in the oxide semiconductor from being reduced.As a result, the oxide semiconductor can be prevented from an undesiredincrease in electron carrier concentration.

The oxygen concentration in a region of the oxide region in each of thesource electrode and the drain electrode decreases toward, for example,the metal region, the region of the oxide region in each of the sourceelectrode and the drain electrode being in contact with the activelayer. This change in the oxygen concentration arises due to oxidizationof the surface of the metal.

The oxygen concentration can be confirmed through TEM-EDX (energydispersive X-ray spectroscopy) or by measuring the depth profile throughX-ray photoelectron spectroscopy.

The metal is preferably a transition metal (simple substance) or analloy thereof in terms of stability as the metal wired line.

The oxides of the oxide region are more preferably a conductive oxide inorder to decrease contact resistance with the active layer. The metalforming the metal regions of the source electrode and the drainelectrode is preferably an element capable of forming a conductive oxideupon forming the oxides.

The metal preferably includes at least one selected from the groupconsisting of Ti, Cu, Ni, Cr, V, Nb, Ta, Mo, and W because the oxygenconcentration of the oxide region can be controlled when the conductiveoxide is formed. The one or more metal oxides forming the oxide regionmay not satisfy stoichiometry.

It is preferable that the oxide regions be formed on the surface of thesource electrode and that of the drain electrode in terms of durabilityto wet-etching in the post-process. Formation of the oxide region overthe whole surface of the source electrode and that of the drainelectrode makes it possible to sufficiently increase a selection ratiobetween etching rates for the source electrode and the drain electrodewith respect to an etching solution for wet-etching to be used in thepost-process. For example, it is known that Ti is subjected to etchingwith a fluorinated acid-based etching solution. However, when a film ofan insulating layer including SiO₂, which is to be subjected to etchingwith fluorinated acid likewise, is formed on the Ti, a sufficientselection ratio of the etching cannot be obtained. However, formation ofthe oxide region makes it possible to increase the selection ratio ofthe etching because a surface of the Ti becomes titanium oxide.

In addition, in terms of close adhesiveness to the bottom ground and theupper layer, when components other than the active layer, which are incontact with the source electrode and the drain electrode, are formed ofoxide, the close adhesiveness to the components other than the activelayer is improved by forming the oxide region on a metal to be formedinto the source electrode and the drain electrode. Therefore, it ispreferable that the oxide region be formed on the metal to be formedinto the source electrode and the drain electrode.

The source electrode and the drain electrode each may have a stackedstructure of a first layer and a second layer, the first layer includingthe metal region and the oxide region and the second layer being formedof a metal.

In that case, the second layer preferably has a higher electricconductivity than that of the metal region of the first layer. As aresult, it is possible to decrease wiring resistance.

The metal of the second layer is not particularly limited and may beappropriately selected depending on the intended purpose. Examples ofthe metal of the second layer include simple substances of thetransition metal, alloys of the transition metal, simple substances ofthe typical metal, and alloys of the typical metal.

The oxides include a transition metal having a positive valence and asubstitutional dopant having a positive valence larger than the positivevalence of the transition metal. The metal preferably includes anelement of the transition metal and an element serving as a dopant withrespect to the oxides. In this case, the oxides are improved in electricconductivity.

Examples of the element serving as a dopant with respect to the oxidesinclude a second metal element such as Group (n+1) elements and Group(n+2) [i.e., Group (n+1) or more element] elements, where the n is theGroup number in the periodic table of a first transition metal element.

The element of the transition metal preferably includes at least oneselected from the group consisting of Ti, V, Nb, Ta, Mo, and W.

The element serving as a dopant with respect to the oxides preferablyincludes at least one selected from the group consisting of V, Nb, Ta,Cr, Mo, W, Mn, and Re.

When the oxide is an oxide undergoing a substitutional doping where apentavalent Nb is substituted with a tetravalent Ti, resistivity can bedecreased compared to the adjacent active layer, which makes it possibleto decrease contact resistivity between the active layer and the sourceelectrode and contact resistivity between the active layer and the drainelectrode.

An amount of the substitutional dopant with respect to the oxides is notparticularly limited and may be appropriately selected depending on theintended purpose. The amount of the substitutional dopant is preferablyfrom 0.01 atom % through 20 atom %, more preferably from 0.1 atom %through 10 atom % relative to an amount of the transition metal in termsof generation of carriers and a scattering factor of the generatedcarriers.

An average thickness of the oxide region is not particularly limited andmay be appropriately selected depending on the intended purpose.However, the average thickness of the oxide region is preferably thinnerthan an average thickness of the metal region.

The average thickness of the oxide region is not particularly limitedand may be appropriately selected depending on the intended purpose, butthe average thickness of the oxide region is preferably from 1 nmthrough 50 nm.

In the bottom contact field-effect transistor, presence of the oxideregion can be confirmed, for example, by simply evaluating the workfunction through photoelectron spectroscopy in the air. In the topcontact field-effect transistor, the presence of the oxide region can beconfirmed through TEM-EDX (energy dispersive X-ray spectroscopy) or bymeasuring the depth profile through X-ray photoelectron spectroscopy.

The work function of the oxide region is larger than the work functionof the metal region.

An average thickness of the source electrode and an average thickness ofthe drain electrode are not particularly limited and may beappropriately selected depending on the intended purpose, but arepreferably from 10 nm through 1 micrometer, more preferably from 50 nmthrough 300 nm.

<Active Layer>

The active layer is formed of an oxide semiconductor.

The active layer is disposed between the source electrode and the drainelectrode.

The active layer positioned between the source electrode and the drainelectrode is a channel region.

The oxide semiconductor preferably includes at least one selected fromthe group consisting of In, Zn, Sn, and Ti.

The oxide semiconductor preferably includes at least one of alkalineearth elements.

The oxide semiconductor preferably includes at least one of rare earthelements.

Examples of the oxide semiconductor include an n-type oxidesemiconductor.

The n-type oxide semiconductor is not particularly limited and may beappropriately selected depending on the intended purpose. However, then-type oxide semiconductor preferably includes at least one selectedfrom the group consisting of indium, zinc, tin, gallium, and titanium.

Examples of the n-type oxide semiconductor include ZnO, SnO₂, In₂O₃,TiO₂, and Ga₂ O₃. Moreover, oxides including a plurality of metals(e.g., In—Zn-based oxides, In—Sn-based oxides, In—Ga-based oxides,Sn—Zn-based oxides, Sn—Ga-based oxides, Zn—Ga-based oxides,In—Zn—Sn-based oxides, In—Ga—Zn-based oxides, In—Sn—Ga-based oxides,Sn—Ga—Zn-based oxides, In—Al—Zn-based oxides, Al—Ga—Zn-based oxides,Sn—Al—Zn-based oxides, In—Hf—Zn-based oxides, and In—Al—Ga—Zn-basedoxides) can be used.

Moreover, it is preferable that the n-type oxide semiconductor undergosubstitutional doping with at least one dopant selected from the groupconsisting of a divalent cation, a trivalent cation, a tetravalentcation, a pentavalent cation, a hexavalent cation, a heptavalent cation,and an octavalent cation, and a valence of the dopant be more than avalence of a metal ion constituting the n-type oxide semiconductor,provided that the dopant is excluded from the metal ion. Here, thesubstitutional doping may be referred to as n-type doping.

In the n-type oxide semiconductor subjected to substitutional doping,part of the metal ions constituting the n-type oxide semiconductor,which is a mother phase, is substituted with a dopant having a highervalence than the valence of the metal ion, and extra electrons generatedbecause of a difference in valence are released to serve as n-typeconductive carriers. In a case where the carrier electrons generated bythe substitutional doping are responsible for semiconductorcharacteristics, the semiconductor characteristics become more stable.The reason for this is as follows. Specifically, the number of carrierelectrons attributed to oxygen vacancies are easily changed byundergoing influences (e.g., oxidation-reduction reactions andadsorption of oxygen onto a surface of the film) when oxygen isexchanged between the semiconductor and the exterior (the atmosphere orthe adjacent layer). Meanwhile, the number of carrier electronsattributed to substitutional doping is relatively free from an influenceof such changes in the state.

Moreover, the number of carrier electrons attributed to substitutionaldoping can be favorably controlled, and a desired carrier concentrationcan be easily achieved, which is one advantage. As described above,oxygen relatively easily moves in and out of the semiconductor, and thusit is difficult to accurately control an amount of oxygen or maintainthe amount of oxygen to a predetermined value. Meanwhile, the number ofthe carrier electrons attributed to the substitutional doping can beeasily and accurately controlled by appropriately selecting a kind ofthe dopant element and a doping amount.

In order to decrease oxygen vacancies in the active layer, it iseffective to introduce more oxygen atoms into the film during the filmformation process of the n-type oxide semiconductor layer (activelayer). For example, in a case where the n-type oxide semiconductorlayer is formed by a sputtering method, a film having less oxygenvacancies can be formed by increasing the oxygen concentration in theatmosphere during the sputtering. Alternatively, in a case where then-type oxide semiconductor layer is formed by coating and baking thecoating liquid, a film having less oxygen vacancies can be formed byincreasing the oxygen concentration in the atmosphere during the baking.

Moreover, an amount of the oxygen vacancies can be decreased dependingon the formulation of the n-type oxide semiconductor. For example,generation of oxygen vacancies can be suppressed by introducing acertain amount of a metal element having high affinity to oxygen (e.g.,Si, Ge, Zr, Hf, Al, Ga, Sc, Y, Ln, and alkaline earth metals).

The kind of the dopant is preferably selected depending on an ionicradius, the number of coordination, and an orbital energy. Aconcentration of the dopant may be appropriately selected depending on amaterial of the mother phase, the kind of the dopant, a site to besubstituted by the dopant, a film formation process, and desiredtransistor characteristics.

Theoretically, the number of electrons generated when one atom issubstituted is a value obtained by subtracting a valence of a metal atomof a mother phase constituting the n-type oxide semiconductor from avalence of a cation (i.e., dopant). That is, the valence of the dopantis preferably large in order to generate the same number of electrons ina smaller doping amount. Moreover, a difference between the valence ofthe dopant and the valence of the metal atom constituting the n-typeoxide semiconductor is preferably larger. When the dopants areexcessively present, crystal structures and alignments of atoms aredisturbed, which prevents carrier electrons from movements. Accordingly,a preferable embodiment is to generate a necessary and sufficient amountof carrier electrons in as small a doping amount as possible.

Moreover, a preferable embodiment is that a selected dopant has an ionicradius close to a radius of an atom to be substituted. This leads toimprovement in substitution efficiency and can prevent an unnecessarydopant not contributing to generation of carriers from deterioratingtransistor characteristics.

An efficiency of generating carriers through doping depends on variousprocess conditions at the time of the production of transistors, andtherefore it is also important to select process conditions that canimprove the carrier generation efficiency. For example, a desiredcarrier concentration can be achieved in a smaller doping amount byappropriately selecting: a temperature of a substrate when a n-typeoxide semiconductor layer is formed by sputtering; a baking temperaturewhen a n-type oxide semiconductor layer is formed by coating and bakinga coating liquid; and a temperature of annealing performed afterformation of the n-type oxide semiconductor layer.

A concentration of the dopant is not particularly limited and may beappropriately selected depending on the intended purpose. Theconcentration of the dopant is preferably from 0.01 mol % through 10 mol%, more preferably from 0.01 mol % through 5 mol %, particularlypreferably from 0.05 mol % through 2 mol % in terms of mobility andrising property. Here, the mol % means a ratio of a molar amount of thedopant to a sum (100%) of a molar amount of a metal element to besubstituted in the semiconductor (that is, a molar amount of the metalion to be substituted with the dopant, which is included in the n-typeoxide semiconductor) and a molar amount of the dopant.

The n-type oxide semiconductor forming an active layer is preferably ina state of monocrystalline or polycrystalline in order forsubstitutional doping to effectively work. Even in a case wherediffraction peaks from the n-type oxide semiconductor are not observedby X-ray diffraction (XRD) and a long-distance order is not present(typically such a state is referred to as an amorphous state), then-type oxide semiconductor preferably has a rigid structure where atomsare aligned orderly in a short distance. The above-described structureis preferable for the following reason. Specifically, in the case wherean oxide semiconductor to be a mother phase is a highly amorphousmaterial, the structure is changed to a locally stable state andcarriers are not generated even after substitutional doping. In the caseof the oxide having the rigid structure, oxygen-coordinating polyhedrons(e.g., WO₆ or InO₆ octahedrons) and their linking manners (e.g., InO₆edge-sharing chains) are maintained, and substitutional dopingeffectively works. In this structure, a state density of tail statesunique to the amorphous state is small and therefore sub-gap absorptionis small. As a result, photodeterioration of the material having theabove structure is excellent than highly amorphous materials.

Doping is similarly effective on the n-type oxide semiconductor even ina monocrystalline or polycrystalline state in which a long-distanceorder is present. In a case where conduction bands are formed with 4s,5s, and 6s bands of heavy metal ions, an influence from grain boundariesis small, and excellent characteristics are obtained even in apolycrystalline state. In a case where a doping amount is excessive andthe dopant is segregated at grain boundaries, it is preferable to lowerthe concentration of the dopant. It is also preferable to perform postannealing at a temperature of from 200 degrees Celsius through 300degrees Celsius in order to improve adhesion and electrical contacts atan interface between the source and drain electrodes and the activelayer. Moreover, annealing may be performed at a higher temperature toenhance crystallinity.

An average thickness of the active layer is not particularly limited andmay be appropriately selected depending on the intended purpose, but theaverage thickness of the active layer is preferably from 5 nm through 1micrometer, more preferably from 10 nm through 0.5 micrometers.

<Gate Insulating Layer>

The gate insulating layer is not particularly limited and may beappropriately selected depending on the intended purpose, so long as thegate insulating layer is a gate insulating layer disposed between thegate electrode and the active layer.

A material of the gate insulating layer is not particularly limited andmay be appropriately selected depending on the intended purpose.Examples of the material of the gate insulating layer include inorganicinsulating materials and organic insulating materials.

Examples of the inorganic insulating materials include silicon oxide,aluminum oxide, tantalum oxide, titanium oxide, yttrium oxide, lanthanumoxide, hafnium oxide, zirconium oxide, silicon nitride, aluminumnitride, and mixtures thereof.

Examples of the organic insulating materials include polyimide,polyamide, polyacrylate, polyvinyl alcohol, and novolac resins.

An average thickness of the gate insulating layer is not particularlylimited and may be appropriately selected depending on the intendedpurpose, but the average thickness of the gate insulating layer ispreferably from 50 nm through 3 micrometers, more preferably from 100 nmthrough 1 micrometer.

<Other Components>

Examples of the other components include a substrate, an insulatinglayer (passivation layer), and an interlayer insulating layer.

<<Substrate>>

A shape, a structure, and a size of the substrate are not particularlylimited and may be appropriately selected depending on the intendedpurpose.

The substrate is not particularly limited and may be appropriatelyselected depending on the intended purpose. Examples of the substrateinclude a glass substrate, a ceramic substrate, a plastic substrate, anda film substrate.

A material of the glass substrate is not particularly limited and may beappropriately selected depending on the intended purpose. Examples ofthe material of the glass substrate include an alkali-free glass and asilica glass.

Materials of the plastic substrate and the film substrate are notparticularly limited and may be appropriately selected depending on theintended purpose. Examples of the materials of the plastic substrate andthe film substrate include polycarbonate (PC), polyimide (PI),polyethylene terephthalate (PET), and polyethylene naphthalate (PEN).

<<Insulating Layer (Passivation Layer)>>

A preferable embodiment is that the transistor also has a configurationwhere the insulating layer (passivation layer) is stacked on thefield-effect transistor including the gate electrode, the sourceelectrode, the drain electrode, the active layer, and the gateinsulating layer. This insulating layer often serves as a so-calledpassivation layer configured to prevent the source electrode, the drainelectrode, and the active layer from directly contacting with oxygen andmoisture in the air and changing their characteristics. In addition, inthe display device including the field-effect transistor, a displayelement including, for example, an emissive layer may be stacked on thetop of the transistor. In this case, the insulating layer also serves asa so-called leveling film, which is configured to absorb leveldifferences derived from a shape of the transistor, to flatten thesurface.

A material of the insulating layer is not particularly limited and maybe appropriately selected depending on the intended purpose. Examples ofthe material of the insulating layer include: materials that havealready widely been used for mass production (e.g., SiO₂, SiON, andSiNx); and organic materials such as polyimide (PI) and fluorine-basedresins.

<<Interlayer Insulating Layer>>

The configurations of the transistor are, for example, a configurationwhere wires for data lines are connected to the source electrode and thedrain electrode and a configuration where data lines are disposed sothat the source electrode and the drain electrode are directly coupledto the active layer. In the aforementioned configurations, it ispreferable that the interlayer insulating layer be formed between thegate electrode and the data line.

A material of the interlayer insulating layer is not particularlylimited and may be appropriately selected depending on the intendedpurpose. Examples of the material of the interlayer insulating layerinclude: materials that have already widely been used for massproduction (e.g., SiO₂, SiON, and SiNx); and organic materials such aspolyimide (PI) and fluorine-based resins. The gate insulating layer canhave the same materials as used for the passivation layer.

A volume resistivity of the interlayer insulating layer is notparticularly limited and may be appropriately selected depending on theintended purpose so long as the interlayer insulating layer is aninsulating film. The volume resistivity of the interlayer insulatinglayer is preferably 1×10¹⁰ Ωcm or more, more preferably 1×10¹² Ωcm ormore, particularly preferably 1×10¹³ Ωcm or more. When the interlayerinsulating layer has a smaller insulation property, malfunction such asleakage and short-circuit may arise.

A formation method of the interlayer insulating layer is notparticularly limited and may be appropriately selected depending on theintended purpose. Examples of the formation method include: (i) a stepof forming a film through sputtering, spin-coating, or slit-coating andpatterning the film through photolithography; and (ii) a method ofdirectly forming a film having a desired shape through a printingprocess such as inkjet printing, nanoimprinting, nozzle printing, orgravure printing.

A method for producing the field-effect transistor is not particularlylimited and may be appropriately selected depending on the intendedpurpose, but is preferably a method for producing the field-effecttransistor of the present disclosure, which will be describedhereinafter.

Hereinafter, a schematic cross-sectional view illustrating one exampleof the field-effect transistor of the present disclosure will beillustrated.

FIG. 1 is a schematic cross-sectional view illustrating one example of afield-effect transistor of the present disclosure.

A field-effect transistor illustrated in FIG. 1 includes a substrate 11,a source electrode 12, a drain electrode 13, an active layer 14, a gateinsulating layer 15, and a gate electrode 16.

The field-effect transistor illustrated in FIG. 1 is a top gate/bottomcontact field-effect transistor field-effect transistor.

The source electrode 12 is formed of a metal region 12A and an oxideregion 12B.

The drain electrode 13 is formed of a metal region 13A and an oxideregion 13B.

In the field-effect transistor illustrated in FIG. 1, the sourceelectrode 12 and the drain electrode 13 are disposed on the insulativesubstrate 11 so as to provide a predetermined interval. The active layer14 is disposed so as to be in contact with a part of the oxide region12B constituting the source electrode 12 and a part of the oxide region13B constituting the drain electrode 13 to form a channel on the activelayer 14. Furthermore, the gate insulating layer 15 is disposed so as tocover the source electrode 12, the drain electrode 13, and the activelayer 14. The gate electrode 16 is disposed on the gate insulating layer15. The oxide region 12B constituting the source electrode 12 is also incontact with the substrate 11 and the gate insulating layer 15. Theoxide region 13B constituting the drain electrode 13 is also in contactwith the substrate 11 and the gate insulating layer 15.

FIG. 2 is a schematic cross-sectional view illustrating another exampleof the field-effect transistor.

A field-effect transistor illustrated in FIG. 2 is a bottom gate/bottomcontact field-effect transistor.

The field-effect transistor illustrated in FIG. 2 includes: a substrate11; a gate electrode 16 disposed on the substrate 11; a gate insulatinglayer 15 disposed on the gate electrode 16; a source electrode 12 and adrain electrode 13 disposed on the gate insulating layer 15; and anactive layer 14 disposed between the source electrode 12 and the drainelectrode 13.

The source electrode 12 is formed of a metal region 12A and an oxideregion 12B.

The drain electrode 13 includes a metal region 13A and an oxide region13B.

The active layer 14 is in contact with a part of the oxide region 12Bconstituting the source electrode 12 and a part of the oxide region 13Bconstituting the drain electrode 13.

The oxide region 12B constituting the source electrode 12 is also incontact with the gate insulating layer 15. The oxide region 13Bconstituting the drain electrode 13 is also in contact with the gateinsulating layer 15.

(Method for Producing Field-Effect Transistor)

A method for producing the field-effect transistor of the presentdisclosure includes at least a step of forming the source electrode, thedrain electrode, and the active layer, and further includes other stepssuch as a step of forming the gate electrode and a step of forming thegate insulating layer if necessary.

The method for producing the field-effect transistor is a method forproducing a field-effect transistor, where the field-effect transistorincludes a gate electrode, which is configured to apply gate voltage;

a source electrode and a drain electrode, which are configured to takeelectric current out; an active layer, which is disposed between thesource electrode and the drain electrode and is formed of an oxidesemiconductor; and a gate insulating layer, which is disposed betweenthe gate electrode and the active layer, the source electrode and thedrain electrode each including a metal region formed of a metal and anoxide region formed of one or more metal oxides, and a part of the oxideregion in each of the source electrode and the drain electrode being incontact with the active layer.

The aforementioned method for producing the field-effect transistor is apreferable method for producing the field-effect transistor of thepresent disclosure.

<Step of Forming Source Electrode, Drain Electrode, and Active Layer>

<<First Aspect>>

One aspect (first aspect) of the step of forming the source electrode,the drain electrode, and the active layer includes a treatment ofoxidizing a surface of a metal layer to form the source electrode andthe drain electrode each having the metal region and the oxide region;and a treatment of forming the active layer so as to be in contact withthe part of the oxide region in each of the source electrode formed andthe drain electrode formed.

The aforementioned treatment of forming the source electrode and thedrain electrode in the first aspect is not particularly limited and maybe appropriately selected depending on the intended purpose, so long asthe treatment is a step of oxidizing a surface of a metal layer to formthe source electrode and the drain electrode. Examples of the treatmentinclude a heat treatment of heating the metal layer and a plasmatreatment of irradiating the metal layer with plasma.

<<Second Aspect>>

Another one aspect (second aspect) of the step of forming the sourceelectrode, the drain electrode, and the active layer includes atreatment of forming the active layer; and a treatment of forming theoxide region so that the part of the oxide region is in contact with theactive layer formed and further forming the metal region on the oxideregion to form the source electrode and the drain electrode eachincluding the oxide region and the metal region.

Examples of the treatment of forming the source electrode and the drainelectrode in the second aspect include a treatment of stacking a metallayer and a conductive oxide layer.

<<Third Aspect>>

Another one aspect (third aspect) of the step of forming the sourceelectrode, the drain electrode, and the active layer includes: atreatment of forming the active layer; a treatment of oxidizing asurface of the metal layer to form the source electrode and the drainelectrode each including the metal region and the oxide region so thatthe part of the oxide region is in contact with the active layer formed.

The first aspect, the second aspect, and the third aspect can beselected depending on the structure of the field-effect transistor.

A bottom contact field-effect transistor has a structure where an uppersurface (outermost surface) of the source electrode and an upper surface(outermost surface) of the drain electrode are in contact with theactive layer. Therefore, any treatment is desirably performed beforeformation of the active layer.

Meanwhile, a top contact field-effect transistor has a structure where abottom surface (undermost surface) of the source electrode and a bottomsurface (undermost surface) of the drain electrode are in contact withthe active layer. Therefore, after formation of the active layer and themetal layer to be formed into the source electrode and the drainelectrode, the heat treatment is performed to form the oxide region atan interface between the active layer and the metal layer. In this case,oxygen for forming the oxide region formed at the interface is mainlysupplied from the oxide semiconductor that forms the active layer.Therefore, oxygen concentration in the oxide semiconductor is designedto be excessive and the oxygen region is formed at the interface throughthe heat treatment, resulting in formation of the stable oxidesemiconductor as the active layer. This makes it possible to obtainsuitable carrier concentration even if the oxide semiconductor isdeprived of oxygen into the metal layer. In this case, the oxidesemiconductor to be the active layer is more preferably an oxidesemiconductor undergoing a substitutional doping in terms of a controlof the carrier concentration of the active layer, as described above.

In the treatment of stacking the metal layer and the conductive oxidelayer, a film of the conductive oxide layer is formed after formation ofthe active layer, followed by formation of a film of the metal layer.The formation of the conductive oxide layer can be appropriatelyselected depending on the intended purpose. Examples of the formation ofthe conductive oxide layer include vacuum processes of the filmformation such as a sputtering method.

In a reactive sputtering method using a metal target, a film can beformed by controlling a flow rate of oxygen upon the film formation tocontrol a concentration of the oxygen in the oxide region. In thereactive sputtering method, the same target can be used to produce athin film having stoichiometry over a wide range of the thin film. Inmost cases, reactive sputtering methods using a metal target, when areactive gas such as oxygen flows at a certain flow rate or higher todischarge electricity with a constant sputtering power, there is aregion at which cathode voltage drastically changes. In order to stablyform a film within this region, a feedback system configured to monitorintensity of plasma emission and the cathode voltage to control the flowrate of the reactive gas in real time is preferably used.

In addition, the metal constituting the conductive oxide layer and themetal constituting the metal layer are the same of a metal.

Alternatively, in order to form the conductive oxide layer, an oxidetarget including the same metal element as that of the metal layer maybe used. In that case, the oxide target for forming the conductive oxidelayer and the metal target for forming the metal layer may separately beused.

In addition to the active layer, another component other than the activelayer is formed of oxides, when oxygen is supplied to form the oxideregions to be a part of the source electrode and a part of the drainelectrode, the active layer and another component other than the activelayer, which are the underlying component, are supplied with sufficientoxygen. Therefore, upon forming the metal region, it is possible tolessen influences given to the underlying component.

A heating temperature of the heating treatment is not particularlylimited and may be appropriately selected depending on the intendedpurpose so long as the surface of the metal layer can be oxidized. Theheating temperature of the heating treatment is preferably from 50degrees Celsius through 400 degrees Celsius, more preferably through 100degrees Celsius from 300 degrees Celsius.

The heating treatment is performed in the air atmosphere so that thesurface of the metal layer or an interface of the active layer and themetal layer can be oxidized.

The plasma treatment is performed in the oxidizing atmosphere, forexample. Examples of the oxidizing atmosphere include atmospheresincluding, for example, oxygen, ozone, and carbon dioxide.

<<Treatment of Forming Active Layer>>

A treatment of forming the active layer is not particularly limited andmay be appropriately selected depending on the intended purpose.Examples of the treatment of forming the active layer include: physicalvapor deposition methods (Physical Vapor Deposition methods) such assputtering and PLD (laser ablation); chemical vapor deposition methodssuch as plasma CVD; solution coating methods such as a sol-gel method;and known film forming methods. A method for pattering the active layeris, for example, a step of the patterning using a shadow mask, a step ofthe patterning through photolithography, and a step of directly forminga film having a desired shape through printing or inkjet.

<Step of Forming Gate Electrode>

The step of forming the gate electrode is not particularly limited andmay be appropriately selected depending on the intended purpose so longas it is a step of forming the gate electrode. Examples of the step offorming the gate electrode include: (i) a step of forming a film througha sputtering method or a dip coating method to pattern the film throughphotolithography; and (ii) a method of directly forming a film having adesired shape through a printing process such as inkjet printing,nanoimprinting, or gravure printing.

<Step of Forming Gate Insulating Layer>

A step of forming the gate insulating layer is not particularly limitedand may be appropriately selected depending on the intended purpose solong as it is a step of forming the gate insulating layer. Examples ofthe step of forming the gate insulating layer include: (i) a step offorming a film through a sputtering method or a dip coating method topattern the film through photolithography; and (ii) a method of directlyforming a film having a desired shape through a printing process such asinkjet printing, nanoimprinting, or gravure printing.

Here, one example of the method for producing the field-effecttransistor of the present disclosure will be described with reference toFIG. 3A to FIG. 3E.

First, a substrate 11 formed of a glass substrate is disposed. Then, ametal film is formed on the substrate 11 through, for example, asputtering method. The formed metal film is patterned throughphotolithography and etching to form a metal layer 2 and a metal layer 3having a desired shape (FIG. 3A). In order to clean a surface of thesubstrate 11 and improve the surface in close adhesiveness, apretreatment such as oxygen plasma, UV ozone, and UV irradiationcleaning is preferably performed before formation of the metal film.

Next, the metal layer 2 and the metal layer 3 are subjected to anoxidation treatment. Examples of the oxidation treatment include a heattreatment step using an oven in the air and a plasma treatment under theoxidizing atmosphere. In the oxidization treatment, the metal layers 2and 3, which are not in contact with the substrate 11, are oxidized.Through the oxidization treatment, the metal layer 2 is formed into thesource electrode 12 including the metal region 12A and the oxide region12B and the metal layer 3 is formed into the drain electrode 13including the metal region 13A and the oxide region 13B (FIG. 3B).

Here, in FIG. 3B, an interface between the metal region 12A and theoxide region 12B and an interface between the metal region 13A and theoxide region 13B are clearly defined. However, in a field-effecttransistor of the present disclosure and a field-effect transistorobtained by a method for producing the field-effect transistor of thepresent disclosure, these interfaces are not necessarily clearly definedso long as it can be confirmed that the metal region and the oxideregion each exist in the source electrode and the drain electrode.

Next, a film of the oxide semiconductor is formed. The formed film ofthe oxide semiconductor is patterned through photolithography andwet-etching to obtain an active layer 14 having a predetermined shape(FIG. 3C).

Then, on the substrate 11, a gate insulating layer 15 is formed througha sputtering method to cover the source electrode 12, the drainelectrode 13, and the active layer 14 (FIG. 3D).

Then, a metal film formed of, for example, aluminum (Al) is formed onthe gate insulating layer 15 through, for example, a sputtering method.The formed metal film is patterned through photolithography and etchingto form a gate electrode 16 having a predetermined shape (FIG. 3E).

Through the above steps, a top gate/bottom contact field-effecttransistor can be produced.

Another one example of a field-effect transistor of the presentdisclosure will be described with reference to one example of a methodfor producing the field-effect transistor.

<<Bottom Gate/Top Contact Field-Effect Transistor>>

A field-effect transistor illustrated in FIG. 4 is a bottom gate/topcontact field-effect transistor.

One example of a method for producing the field-effect transistor willbe described hereinafter.

First, a gate electrode 16 is formed on a substrate 11.

Next, a gate insulating layer 15 is formed on the substrate 11 and thegate electrode 16.

Then, an active layer 14 is formed on the gate insulating layer 15.

Then, a source electrode 12 and a drain electrode 13 are formed on thegate insulating layer 15 and the active layer 14. The source electrode12 is formed of a metal region 12A and an oxide region 12B. The drainelectrode 13 is formed of a metal region 13A and an oxide region 13B.

The source electrode 12 and the drain electrode 13 are formed through,for example, the following method. An oxide film is formed on the gateinsulating layer 15 and the active layer 14 and then a metal film isformed on the oxide film, which is subjected to etching at one time. Asa result, the oxide film and the metal film are divided to form oxideregions 12B and 13B and metal regions 12A and 13A at one time. Then, thesource electrode 12 and the drain electrode 13 are formed.

Formation of the oxide film can be performed through, for example, asputtering method. In a reactive sputtering method using a metal target,a film can be formed by controlling a flow rate of oxygen upon formingthe film and by controlling a concentration of the oxygen in the oxideregion. In addition, the metal constituting the oxide film and the metalconstituting the metal film are the same of a metal.

As described above, a bottom gate/top contact field-effect transistorillustrated in FIG. 4 can be obtained.

<<Top Gate/Top Contact Field-Effect Transistor>>

Field-effect transistors illustrated in FIG. 5 and FIG. 7 are topgate/top contact field-effect transistors.

One example of a method for forming these field-effect transistors willbe described hereinafter.

First, an active layer 14 is formed on a substrate 11.

Next, a source electrode 12 and a drain electrode 13 are formed on thesubstrate 11 and the active layer 14. The source electrode 12 is formedof a metal region 12A and an oxide region 12B. The drain electrode 13 isformed of a metal region 13A and an oxide region 13B.

The source electrode 12 and the drain electrode 13 are formed through,for example, the following method. An oxide film is formed on thesubstrate 11 and the active layer 14 and then a metal film is formed onthe oxide film, which is subjected to etching at one time. As a result,the oxide film and the metal film are divided to form oxide regions 12Band 13B and metal regions 12A and 13A at one time. Then, the sourceelectrode 12 and the drain electrode 13 are formed.

Formation of the oxide film can be performed through, for example, asputtering method. In a reactive sputtering method using a metal target,a film can be formed by controlling a flow rate of oxygen upon formingthe film and by controlling a concentration of the oxygen in the oxideregion. In addition, the metal constituting the oxide film and the metalconstituting the metal film are the same of a metal.

Next, a gate insulating layer 15 is formed on the source electrode 12,the drain electrode 13, and the active layer 14.

Then, a gate electrode 16 is formed on the gate insulating layer 15.

As described above, the top gate/top contact field-effect transistorillustrated in FIG. 5 can be obtained.

Furthermore, an interlayer insulating layer 17 is formed on the gateinsulating layer 15 and the gate electrode 16.

Next, through holes are formed so as to reach the source electrode 12and the drain electrode 13 in the gate insulating layer 15 and theinterlayer insulating layer 17. Then, data lines 18 are formed.

As described above, the top gate/top contact field-effect transistorillustrated in FIG. 7 can be obtained.

<<Top Gate/Top Contact Field-Effect Transistor>>

A field-effect transistor illustrated in FIG. 6 is a top gate/topcontact field-effect transistor.

One example of a method for producing the field-effect transistor willbe described hereinafter.

First, an active layer 14 is formed on a substrate 11.

Next, a source electrode 12 and a drain electrode 13 are formed on thesubstrate 11 and the active layer 14. The source electrode 12 is formedof a metal region 12A and an oxide region 12B. The drain electrode 13 isformed of a metal region 13A and an oxide region 13B.

The source electrode 12 and the drain electrode 13 are formed through,for example, the following method. A metal film is formed on thesubstrate 11 and the active layer 14, which is subjected to a heattreatment. As a result, oxide regions 12B and 13B are formed at aninterface between the active layer 14 and the metal film and on thesurface of the metal film. That is, the oxide regions 12B and 13B areformed around the metal regions 12A and 13A to form the source electrode12 and the drain electrode 13 as illustrated in FIG. 6.

The formation of the metal film can be performed via a metal maskthrough a sputtering method.

Then, a gate insulating layer 15 is formed on the source electrode 12,the drain electrode 13, and the active layer 14.

Then, a gate electrode 16 is formed on the gate insulating layer 15.

As described above, a top gate/top contact field-effect transistorillustrated in FIG. 6 can be obtained.

A field-effect transistor illustrated in FIG. 8 is a top gate/topcontact field-effect transistor.

One example of a method for producing the field-effect transistor willbe described hereinafter.

First, an active layer 14 is formed on a substrate 11.

Next, a gate insulating layer 15 is formed on the substrate 11 and theactive layer 14.

Then, a gate electrode 16 is formed on the gate insulating layer 15.

Then, an interlayer insulating layer 17 is formed on the gate insulatinglayer 15 and the gate electrode 16.

Then, an area to be the source electrode 12 and an area to be the drainelectrode 13, each of which passes in the gate insulating layer 15 andthe interlayer insulating layer 17, are subjected to etching until theactive layer 14 is exposed.

Then, a gate electrode 12 and a drain electrode 13 are formed on theexposed active layer 14.

The source electrode 12 and the drain electrode 13 are formed through,for example, the following method. An oxide film is formed on thesurfaces of the interlayer insulating layer 17, the gate insulatinglayer 15, and the active layer 14. Then, a metal film is formed on theoxide film, which is subjected to etching at one time. As a result, theoxide film and the metal film are divided to form oxide regions 12B and13B and metal regions 12A and 13A at one time. Then, the sourceelectrode 12 and the drain electrode 13 are formed.

Formation of the oxide film can be performed through, for example, asputtering method. In a reactive sputtering method using a metal target,a film can be formed by controlling a flow rate of oxygen upon formingthe film and by controlling a concentration of the oxygen in the oxideregion. In addition, the metal constituting the oxide film and the metalconstituting the metal film are the same of a metal.

As described above, a top gate/top contact field-effect transistorillustrated in FIG. 8 can be obtained.

As a modification example of FIG. 6, a bottom gate/top contactfield-effect transistor is illustrated in FIG. 9.

As a modification example of FIG. 7, a bottom gate/top contactfield-effect transistor is illustrated in FIG. 10.

As a modification example of FIG. 8, a bottom gate/top contactfield-effect transistor is illustrated in FIG. 11.

As one example of a method for producing these field-effect transistors,only different points from the method for producing the top gate/topcontact field-effect transistor will be extracted and described below.

First, a gate electrode 16 is formed on a substrate 11.

Next, a gate insulating layer 15 is formed on the substrate 11 and thegate electrode 16.

Then, an active layer 14 is formed on the gate insulating layer 15.

Hereinafter, the interlayer insulating layer 17, the source electrode12, and the drain electrode 13 are formed in the same manner as in eachmethod for forming the field-effect transistor of FIGS. 6 to 8.

As described above, the bottom gate/top contact field-effect transistorsof FIGS. 9 to 11, which are modification examples of FIGS. 6 to 8, canbe obtained.

(Display Element)

A display element of the present disclosure includes at least an opticalcontrol element and a driving circuit configured to drive the opticalcontrol element. The display element further includes other componentsaccording to the necessity.

<Optical Control Element>

The optical control element is not particularly limited and may beappropriately selected depending on the intended purpose, so long as theoptical control element is an element configured to control a lightoutput according to a driving signal. The optical control elementpreferably includes an organic electroluminescent (EL) element, anelectrochromic (EC) element, a liquid crystal element, anelectrophoretic element, or an electrowetting element.

<Driving Circuit>

The driving circuit is not particularly limited and may be appropriatelyselected depending on the intended purpose, so long as the drivingcircuit includes the semiconductor element of the present disclosure.

<Other Components>

The other components are not particularly limited and may beappropriately selected depending on the intended purpose.

The display element of the present disclosure includes the semiconductorelement (for example, the field-effect transistor). Therefore,unevenness between elements is small. Moreover, even when the displayelement changes over time, the driving transistor can be driven atconstant gate voltage, resulting in long lifetime of the elements.

(Image Display Device)

An image display device of the present disclosure includes at least aplurality of display elements, a plurality of wires, and a displaycontrol device. The image display device further includes othercomponents according to the necessity.

<Display Element>

The display element is not particularly limited and may be appropriatelyselected depending on the intended purpose, so long as the displayelement is the display element of the present disclosure arranged in aform of matrix.

<Wired Line>

The wired line is not particularly limited and may be appropriatelyselected depending on the intended purpose, so long as the wired line isa wired line configured to individually apply gate voltage and imagedata signal to the field-effect transistor in the display element.

<Display Control Device>

The display control device is not particularly limited and may beappropriately selected depending on the intended purpose, so long as thedisplay control device is a device configured to individually controlthe gate voltage and the signal voltage of the field-effect transistorsvia a plurality of the wires correspondingly to the image data.

<Other Components>

The other components are not particularly limited and may beappropriately selected depending on the intended purpose.

An image display device of the present disclosure includes the displayelement of the present disclosure. Therefore, the image display devicehas long lifetime and is stably driven.

The image display device of the present disclosure can be used as adisplay unit in mobile information devices (e.g., mobile phones,portable music players, portable video players, electronic books, andpersonal digital assistants (PDAs)) and camera devices (e.g., stillcameras and video cameras). The image display device can also be usedfor display units of various pieces of information in transportationsystems (e.g., cars, aircraft, trains, and ships). Furthermore, theimage display device can be used for display units of various pieces ofinformation in measuring devices, analysis devices, medical equipment,and advertising media.

(System)

A system of the present disclosure includes at least the image displaydevice of the present disclosure and an image-data-generating device.

The image-data-generating device is configured to generate image databased on image information to be displayed and to output the image datato the image display device.

Because the system of the present disclosure includes the image displaydevice of the present disclosure, image information with high definitioncan be displayed.

The image display device of the present disclosure will next bedescribed hereinafter.

The image display device of the present disclosure can be obtained byemploying configurations described in paragraphs 0059 to 0060 and FIGS.2 and 3 of Japanese Unexamined Patent Application Publication No.2010-074148.

One example of the embodiment of the present disclosure will next bedescribed with reference to the figures.

FIG. 12 is a diagram for presenting a display in which display elementsare arranged in a form of matrix. As illustrated in FIG. 12, the displayincludes “n” scanning lines (X0, X1, X2, X3, . . . Xn−2, Xn−1) arrangedalong the X axis direction at constant intervals, “m” data lines (Y0,Y1, Y2, Y3, . . . Ym−1) arranged along the Y axis direction at constantintervals, and “m” current supply lines (Y0i, Y1i, Y2i, Y3i, . . .Ym−1i) arranged along the Y axis direction at constant intervals. Here,meanings of reference numerals (for example, X1 and Y1) are in commonthroughout FIGS. 13, 17, 18, and 19.

Therefore, the display element 302 can be identified by the scanninglines and the data lines.

FIG. 13 is a schematic structural view illustrating one example of thedisplay element of the present disclosure.

As illustrated as one example in FIG. 13, the display element includesan organic electroluminescent (EL) element 350 and a drive circuit 320configured to allow the organic EL element 350 to emit light. That is, adisplay 310 is an organic EL display of a so-called active matrixsystem. Moreover, the display 310 is a 32-inch display adaptable tocolors. A size of the display 310 is not limited to 32 inches.

The drive circuit 320 in FIG. 13 will be described.

The drive circuit 320 includes two field-effect transistors 10 and 20and a capacitor 30.

A field-effect transistor 10 serves as a switching element. A gateelectrode G of the field-effect transistor 10 is coupled to apredetermined scanning line and a source electrode S of the field-effecttransistor 10 is coupled to a predetermined data line. Moreover, a drainelectrode D of the field-effect transistor 10 is coupled to one terminalof the capacitor 30.

The field-effect transistor 20 is configured to supply electric currentto the organic EL element 350. The gate electrode G of the field-effecttransistor 20 is coupled to the drain electrode D of the field-effecttransistor 10. The drain electrode D of the field-effect transistor 20is coupled to the anode of the organic EL element 350 and a sourceelectrode S of the field-effect transistor 20 is coupled to apredetermined current supply line.

The capacitor 30 is configured to memorize the state of the field-effecttransistor 10; i.e., data. The other terminal of the capacitor 30 iscoupled to a predetermined current supply line.

When the field-effect transistor 10 turns into the state of “on”, imagedata are stored in the capacitor 30 via the signal line Y2. Even afterturning the field-effect transistor 10 into the state of “off”, theorganic EL element 350 is driven by maintaining the “on” state of thefield-effect transistor 20 corresponding to the image data.

FIG. 14 presents one example of a positional relationship between anorganic EL element 350 and a field-effect transistor 20 serving as adrive circuit in a display element. Here, the organic EL element 350 isdisposed next to the field-effect transistor 20. Note that, afield-effect transistor and a capacitor (not illustrated) are alsoformed on the same substrate.

A passivation film is suitably disposed on or above the active layer 22,although the passivation film is not illustrated in FIG. 14. A materialof the passivation film may be appropriately selected from SiO₂, SiNx,Al₂O₃, and fluoropolymers.

As illustrated in FIG. 15, for example, the organic EL element 350 maybe disposed on the field-effect transistor 20. In the case of thisstructure, the gate electrode 26 is required to have transparency.Therefore, a conductive transparent oxide (e.g., ITO, In₂O₃, SnO₂, ZnO,Ga-added ZnO, Al-added ZnO, and Sb-added SnO₂) is used for the gateelectrode 26. Note that, reference numeral 360 is an interlayerinsulating film (a leveling film). Polyimide or acrylic resins can beused for the insulating film.

In FIGS. 14 and 15, the field-effect transistor 20 includes a substrate21, an active layer 22, a source electrode 23, a drain electrode 24, agate insulating layer 25, and a gate electrode 26. An organic EL element350 includes a cathode 312, an anode 314, and an organic EL thin filmlayer 340.

FIG. 16 is a schematic structural view illustrating one example of anorganic EL element.

In FIG. 16, an organic EL element 350 includes a cathode 312, an anode314, and an organic EL thin film layer 340.

A material of the cathode 312 is not particularly limited and may beappropriately selected depending on the intended purpose. Examples ofthe material include aluminium (Al), magnesium (Mg)-silver (Ag) alloy,aluminium (Al)-lithium (Li) alloy, and indium tin oxide (ITO). Notethat, the magnesium (Mg)-silver (Ag) alloy becomes a high-reflectiveelectrode if having a sufficient thickness, and an extremely thin film(less than about 20 nm) of the Mg—Ag alloy becomes a semi-transparentelectrode. In the figure, light is taken out from the side of the anode.However, light can be taken out from the side of the cathode when thecathode is a transparent electrode or a semi-transparent electrode.

A material of the anode 314 is not particularly limited and may beappropriately selected depending on the intended purpose. Examples ofthe material include indium tin oxide (ITO), indium zinc oxide (IZO),and silver (Ag)-neodymium (Nd) alloy. Note that, in a case where asilver alloy is used, the resultant electrode becomes a high-reflectiveelectrode, which is suitable for taking light out from the side of thecathode.

The organic EL thin film layer 340 includes an electron transportinglayer 342, a light emitting layer 344, and a hole transporting layer346. The electron transporting layer 342 is coupled to a cathode 312 andthe hole transporting layer 346 is coupled to an anode 314. The lightemitting layer 344 emits light when a predetermined voltage is appliedbetween the anode 314 and the cathode 312.

The electron transporting layer 342 and the light emitting layer 344 mayform a single layer. Moreover, an electron injecting layer may bedisposed between the electron transporting layer 342 and the cathode312. Furthermore, a hole injecting layer may be disposed between thehole transporting layer 346 and the anode 314.

The above-described organic EL element is a so-called “bottom emission”organic EL element, in which light is taken out from the side of thesubstrate (the bottom side in FIG. 16). However, the organic EL elementmay be a “top emission” organic EL element, in which light is taken outfrom the opposite side to the substrate (the bottom side in FIG. 16).

FIG. 17 is a schematic structural view illustrating another example ofthe image display device of the present disclosure.

In FIG. 17, the image display device includes display elements 302,wires (including scanning lines, data lines, and current supply lines),and a display control device 400.

The display control device 400 includes an image-data-processing circuit402, a scanning-line-driving circuit 404, and a data-line-drivingcircuit 406.

The image-data-processing circuit 402 determines brightness of aplurality of display elements 302 in the display based on output signalsof an image output circuit.

The scanning-line-driving circuit 404 individually applies voltage to“n” scanning lines according to the instructions of theimage-data-processing circuit 402.

The data-line-driving circuit 406 individually applies voltage to “m”data lines according to the instructions of the image-data-processingcircuit 402.

In the above embodiment, a case where the optical control element is anorganic EL element has been described, but the present disclosure is notlimited to the above. For example, the optical control element may be anelectrochromic element. In this case, the display is an electrochromicdisplay.

The optical control element may be a liquid crystal element. In thiscase, the display is a liquid crystal display. As illustrated in FIG.18, it is not necessary to provide a current supply line for a displayelement 302′. As illustrated in FIG. 19, the drive circuit 320′ may beproduced with one field-effect transistor 40, which is similar to eachof the field-effect transistors 10 and 20. In the field-effecttransistor 40, a gate electrode G is coupled to a predetermined scanningline and a source electrode S is coupled to a predetermined data line.Moreover, a drain electrode D is coupled to a capacitor 361 and a pixelelectrode of a liquid crystal element 370.

The optical control element may be an electrophoretic element, aninorganic EL element, or an electrowetting element.

As described above, a case where the system of the present disclosure isa television device has been described, but the system of the presentdisclosure is not limited to the television device. The system is notparticularly limited, so long as the system includes the image displaydevice serving as a device configured to display images and information.For example, the system may be a computer system in which a computer(including a personal computer) is coupled to the image display device.

A system of the present disclosure includes the display element of thepresent disclosure. Therefore, the system has long lifetime and isstably driven.

EXAMPLES

The present disclosure will next be described by way of Examples, butthe present disclosure should not be construed as being limited to theseExamples.

Example 1

<Production of Field-Effect Transistor>

In Example 1, a top gate/bottom contact field-effect transistor aspresented in FIG. 1 was produced. Here, numerical references presentedin the following Examples correspond to the numerical references in FIG.1 and FIGS. 3A to 3E.

<<Formation of Source Electrode and Drain Electrode>>

Formation of Source Electrode Precursor and Drain Electrode Precursor

An Al film to be the second layer was formed on a substrate 11 through asputtering method so as to have a thickness of 100 nm. Then, a Mo filmto be the first layer was formed on the Al film so as to have athickness of 30 nm.

Next, resist patterns were formed through photolithography on the formedAl/Mo film and the resultant was subjected to etching to form, on thesubstrate 11, a metal layer 2 and a metal layer 3 each of which has apredetermined shape.

—Oxidation Treatment—

The formed metal layer 2 and the formed metal layer 3 were subjected toheat treatment at 200 degrees Celsius with an oven under the atmosphereto form oxide regions 12B and 13B.

<<Formation of Active Layer>>

Next, a film of MgIn₂O₄ was formed so as to have a film thickness of 50nm through an RF magnetron sputtering method on a region disposed acrossthe source electrode and the drain electrode on the substrate. Asdescribed above, the surfaces of the source electrode and the drainelectrode include oxygen, and the active layer is stacked thereon.Therefore, the oxygen-including regions of the electrodes are in contactwith the active layer. When the active layer was formed throughsputtering, a poly-crystalline sintered body having a constitution ofMgIn₂O₄ was used as a target. An argon gas and an oxygen gas wereintroduced as sputtering gasses. A total pressure was fixed to 1.1 Paand oxygen concentration was set to 2.5% by volume. On the formedMgIn₂O₄ film, resist patterns were formed through photolithography andthe resultant was subjected to etching to form an active layer 14 havinga predetermined shape.

<<Formation of Gate Insulating Layer>>

Next, through a sputtering method, a film of SiO₂ was formed so as tohave a thickness of 200 nm to form a gate insulating layer 15.

<<Formation of Gate Electrode>>

Finally, an Al film was formed on the gate insulating layer 15 through asputtering method so as to have a thickness of 100 nm. On the formed Alfilm, resist patterns were formed through photolithography and theresultant was subjected to etching to form a gate electrode 16 having apredetermined shape. Subsequently, using an oven, the film was subjectedto an annealing treatment for 1 hour at 300 degrees Celsius in the air.This annealing treatment is generally performed in order to improvetransistor characteristics by decreasing interface defect densitybetween the active layer and the gate insulating layer.

As described above, a top gate/bottom contact field-effect transistorpresented in FIG. 1 was completed.

(Production of Element for Measuring Work Function)

In order to measure work function, an element for measuring workfunction was obtained in the same manner as in the <<Formation of sourceelectrode and drain electrode>> except that a metal layer, on which asurface of the metal layer was oxidized, was formed on a glasssubstrate.

(Evaluation of Work Function)

In the obtained element for measuring work function, a photoelectronspectroscopy AC-2 (available from Riken Keiki Co., Ltd.) was used tomeasure the surface-oxidized metal layer for work function in the air.The obtained work function was presented in Table 1. In Table 1,configurations of the source electrode and the drain electrode arepresented. However, when the source electrode and the drain electrodehave a single layer, the electrodes are presented as the “first layer”for the sake of convenience.

(Electric Characteristics)

The field-effect transistor obtained in Example 1 was evaluated fortransistor performances using a semiconductor parameter-analyzer device(available from Agilent Technologies, semiconductor parameter analyzerB1500). Specifically, the source/drain electric current (Ids) and thegate electric current |Ig| were measured by changing gate voltage (Vg)from −15 V to +15 V with the source/drain voltage (Vds) being 10 V toevaluate electric current-voltage characteristics.

As a result of evaluation of the transistor performances, favorabletransistor characteristics were obtained. A ratio (on/off ratio) of thesource/drain electric current (Ids) of the on-state (for example, Vg=15V) to the source/drain electric current (Ids) of the off-state (forexample, Vg=−15 V) of the transistor was calculated and was presented inTable 1.

Example 2

<Production of Field-Effect Transistor>

In Example 2, a top gate/bottom contact field-effect transistor wasproduced in the same manner as in Example 1 except that the —Formationof source electrode precursor and drain electrode precursor—, theoxidation treatment in the “formation of source electrode and drainelectrode”, and the <<Formation of active layer>> were changed to thebelow-described methods.

The same evaluations as described in Example 1 were performed. Resultsare presented in Table 1.

<<Formation of Source Electrode and Drain Electrode>>

—Formation of Source Electrode Precursor and Drain Electrode Precursor—

A metal layer 2 and a metal layer 3 were formed in the same manner as inthe production steps of the field-effect transistor in Example 1 exceptthat the target used for forming the first layer was changed to eachtarget presented in Table 1.

—Oxidation Treatment—

The formed metal layer 2 and metal layer 3 were subjected to plasmatreatment in an oxidizing atmosphere under the following conditions toform oxide regions 12B and 13B.

The plasma treatment under the oxidizing atmosphere was performed underthe following conditions: ultimate vacuum within the chamber: 10 Pa orless; oxygen flow rate: 50 sccm; and supplied power: 500 W.

<<Formation of Active Layer>>

An active layer was formed in the same manner as in Example 1 exceptthat the target of the Example 1 was changed to each target presented inTable 1.

Example 3

<Production of Field-Effect Transistor>

In Example 3, a top gate/bottom contact field-effect transistor wasproduced in the same manner as in Example 1 except that the —Formationof source electrode precursor and drain electrode precursor— and the“formation of active layer” in Example 1 were changed.

The same evaluations as described in Example 1 were performed. Resultsare presented in Table 1.

—Formation of Source Electrode Precursor and Drain Electrode Precursor—

A metal layer 2 and a metal layer 3 were formed in the same manner as inthe production steps of the field-effect transistor in Example 1 exceptthat the target used for forming the first layer was changed to eachtarget presented in Table 1.

<<Formation of Active Layer>>

Next, on a region disposed across the source electrode and the drainelectrode on the substrate, a film of W-doped MgIn₂O₄ was formed througha RF magnetron sputtering method so as to have a film thickness of 50nm. As described above, the surfaces of the source electrode and thedrain electrode include oxygen, and the active layer is stacked thereon.Therefore, the oxygen-including regions of the electrodes are configuredto be in contact with the active layer. When the active layer was formedthrough sputtering, a polycrystalline sintered body having aconstitution of MgIn_(1.99)W_(0.01)O₄ was used as a target. An argon gasand an oxygen gas were introduced as sputtering gasses. A total pressurewas fixed to 1.1 Pa and oxygen concentration was set to 10% by volume.On the formed W-doped MgIn₂O₄, resist patterns were formed throughphotolithography and the resultant was subjected to etching to form anactive layer 14 having a predetermined shape. In the thus-obtainedactive layer, In in MgIn₂O₄ underwent substitutional doping with W in aconcentration of 0.5 mol %.

Example 4

<Production of Field-Effect Transistor>

In Example 4, a top gate/bottom contact field-effect transistor wasproduced in the same manner as in Example 3 except that the —Formationof source electrode precursor and drain electrode precursor— in Example3 was changed to the below-described method.

The same evaluations as described in Example 1 were performed. Resultsare presented in Table 1.

—Formation of Source Electrode Precursor and Drain Electrode Precursor—

A metal layer 2 and a metal layer 3 were formed in the same manner as inthe production steps of the field-effect transistor in Example 3 exceptthat the target used for forming the first layer was changed to eachtarget presented in Table 1.

Example 5

<Production of Field-Effect Transistor>

In Example 5, a top gate/bottom contact field-effect transistor wasproduced in the same manner as in Example 2 except that the —Formationof source electrode precursor and drain electrode precursor— and the“formation of active layer” in Example 2 were changed to thebelow-described methods.

The same evaluations as described in Example 1 were performed. Resultsare presented in Table 1.

—Formation of Source Electrode Precursor and Drain Electrode Precursor—

A metal layer 2 and a metal layer 3 were formed in the same manner as inthe production steps of the field-effect transistor in Example 1 exceptthat the target used for forming the first layer was changed to eachtarget presented in Table 1.

<<Formation of Active Layer>>

A film of an active layer was formed in the same manner as in theproduction steps of the field-effect transistor in Example 2 except thatthe sintered body target for forming the active layer and theconcentration of oxygen (i.e., sputtering gas) were changed as describedin Table 1.

Example 6

<Production of Field-Effect Transistor>

In Example 6, a top gate/bottom contact field-effect transistor wasproduced in the same manner as in Example 5 except that the -Formationof source electrode precursor and drain electrode precursor- in Example5 was changed to the below-described method.

The same evaluations as described in Example 1 were performed. Resultsare presented in Table 1.

—Formation of Source Electrode Precursor and Drain Electrode Precursor—

A metal layer 2 and a metal layer 3 were formed in the same manner as inthe production steps of the field-effect transistor in Example 1 exceptthat the target used for forming the first layer was changed to eachtarget presented in Table 1.

Example 7

<Production of Field-Effect Transistor>

In Example 7, a top gate/top contact field-effect transistor presentedin FIG. 5 was produced. Here, numerical references presented in thefollowing Examples correspond to the numerical references in FIG. 5.

Regarding the electric characteristics, the same evaluation as describedin Example 1 was performed and results are presented in Table 2. Thework function was not evaluated because the oxide region was disposedunder the metal region.

<<Formation of Active Layer>>

On the substrate, a film of W-doped Y_(0.6)In_(1.4)O₃ was formed througha RF magnetron sputtering method so as to have a film thickness of 50nm. When the active layer was formed through sputtering, apolycrystalline sintered body having a constitution ofY_(0.6)In_(1.39)W_(0.01)O₃ was used as a target. An argon gas and anoxygen gas were introduced as sputtering gasses. A total pressure wasfixed to 1.1 Pa and oxygen concentration was set to 10% by volume. Onthe formed W-doped Y_(0.6)In_(1.4)O₃, resist patterns were formedthrough photolithography and the resultant was subjected to etching toform an active layer 14 having a predetermined shape. In thethus-obtained active layer, In in Y_(0.6)In_(1.39)W_(0.01)O₃ underwentsubstitutional doping with W in a concentration of 0.7 mol %.

<<Formation of source electrode and drain electrode>>

—Stacking Treatment of Source Electrode and Drain Electrode—

On the substrate 11 and the active layer 14, a Ti—Nb alloy target(Ti:Nb=98:2 (atomic ratio)) was used to continuously deposit films of anoxide region 12B, an oxide region 13B, a metal region 12A, and a metalregion 13A through a reactive sputtering method to form a sourceelectrode 12 and a drain electrode 13. The above method will bedescribed hereinafter.

An argon gas and an oxygen gas were introduced as sputtering gasses. ANb-doped titanium oxide film to be oxide regions was formed so as tohave a thickness of 5 nm. A total pressure was fixed to 1.1 Pa andoxygen concentration was set to 10% by volume. Here, the titanium oxidefilm has good electric conductivity even if it does not satisfy thestoichiometric composition ratio. Subsequently, using an argon gas aloneintroducing as a sputtering gas, the same Ti—Nb alloy target was used toform a film of Ti—Nb so as to have a thickness of 100 nm. A sufficientpre-sputtering was performed so as to make oxidation states of thetarget surface uniform before each of the regions was formed.

Patterning was performed through a metal mask to form each film. Then,an oxide region 12B, an oxide region 13B, a metal region 12A, and ametal region 13A each having a predetermined shape were formed on thesubstrate 11.

<<Formation of Gate Insulating Layer>>

Next, a gate insulating layer 15 was formed by forming a film of SiO₂through a sputtering method so as to have a thickness of 200 nm.

<<Formation of Gate Electrode>>

Finally, an Al film was formed on the gate insulating layer 15 through asputtering method so as to have a thickness of 100 nm. On the formed Alfilm, resist patterns were formed through photolithography and theresultant was subjected to etching to form a gate electrode 16 having apredetermined shape. Subsequently, the film was subjected to anannealing treatment using an oven for 1 hour at 300 degrees Celsius inthe air. This annealing treatment is generally performed in order toimprove transistor characteristics by decreasing interface defectdensity between the active layer and the gate insulating layer.

As described above, a top gate/bottom contact field-effect transistorpresented in FIG. 5 was completed.

Example 8

<Production of Field-Effect Transistor>

In Example 8, a top gate/top contact field-effect transistor wasproduced in the same manner as in Example 7 except that the —Stackingtreatment of source electrode and drain electrode— and the <<Formationof active layer>> in Example 7 were changed to the below-describedmethods.

Regarding the electric characteristics, the same evaluation as describedin Example 1 was performed and results are presented in Table 2. Thework function was not evaluated because the oxide region was disposedunder the metal region.

—Stacking Treatment of Source Electrode and Drain Electrode—

An oxide region 12B, an oxide region 13B, a metal region 12A, and ametal region 13A were formed in the same manner as in the productionsteps of the field-effect transistor in Example 7 except that the targetwas changed to a V-W alloy target (V:W=98:2 (atomic ratio)) to be oxideregions to form an oxide region 12B, an oxide region 13B, a metal region12A, and a metal region 13A.

<<Formation of Active Layer>>

A film of an active layer was formed in the same manner as in theproduction steps of the field-effect transistor of Example 7 except thatthe sintered body target for forming the active layer was changed asdescribed in Table 2.

Example 9

<Production of Field-Effect Transistor>

In Example 9, a top gate/top contact field-effect transistor wasproduced in the same manner as in Example 7 except that the —Stackingtreatment of source electrode and drain electrode— in Example 7 waschanged to the below-described method.

Regarding the electric characteristics, the same evaluation as describedin Example 1 was performed and results are presented in Table 2. Thework function was not evaluated because the oxide region was disposedunder the metal region.

—Stacking Treatment of Source Electrode and Drain Electrode—

In the production steps of the field-effect transistor in Example 7, afilm of Nb-doped TiO₂ was formed through a DC magnetron sputteringmethod so as to have a thickness of 5 nm. When the oxide regions wereformed through sputtering, a polycrystalline sintered body having aconstitution of Ti_(0.9)Nb_(0.1)O₂ was used as a target. An argon gasand an oxygen gas were introduced as sputtering gasses. A total pressurewas fixed to 1.1 Pa and oxygen concentration was set to 1% by volume.

Subsequently, a film of Ti—Nb to be metal regions was formed so as tohave a thickness of 100 nm using a Ti—Nb alloy target. An argon gasalone was used as a sputtering gas. Patterning was performed through ametal mask to form each film. Then, an oxide region 12B, an oxide region13B, a metal region 12A, and a metal region 13A each having apredetermined shape were formed.

Example 10

<Production of Field-Effect Transistor>

In Example 10, a top gate/top contact field-effect transistor presentedin FIG. 6 was produced. Here, numerical references presented in thefollowing Examples correspond to the numerical references in FIG. 6. Thesame evaluations as described in Example 1 were performed. Results arepresented in Table 3.

<<Formation of Active Layer>>

Next, on a substrate, a film of W-doped MgIn₂O₄ was formed through a RFmagnetron sputtering method so as to have a film thickness of 50 nm.When the active layer was formed through sputtering, a polycrystallinesintered body having a constitution of MgIn_(1.99)W_(0.01)O₄ was used asa target. An argon gas and an oxygen gas were introduced as sputteringgasses. A total pressure was fixed to 1.1 Pa and oxygen concentrationwas set to 50% by volume to form an excessively oxidized active layer.On the formed W-doped MgIn₂O₄, resist patterns were formed throughphotolithography and the resultant was subjected to etching to form anactive layer 14 having a predetermined shape. In the thus-obtainedactive layer, In in MgIn₂O₄ underwent substitutional doping with W in aconcentration of 0.5 mol %.

<<Formation of Source Electrode and Drain Electrode>>

—Formation of Source Electrode Precursor and Drain Electrode Precursor—

On the substrate 11 and the active layer 14, a film of Mo was formed soas to have a thickness of 100 nm through a sputtering method. Patterningwas performed through a metal mask to form each film. A metal layer(source electrode precursor) and a metal layer (drain electrodeprecursor) each having a predetermined shape were formed on thesubstrate 11 and the active layer 14.

—Oxidation Treatment—

The metal layer (source electrode precursor) and the metal layer (drainelectrode precursor) formed were subjected to a heat treatment in anoven in the air at 200 degrees Celsius to form oxide regions 12B and 13Bon regions that are in contact with the active layer.

<<Formation of Gate Insulating Layer>>

Next, a gate insulating layer 15 was formed by forming a film of SiO₂ soas to have a thickness of 200 nm through a sputtering method.

<<Formation of Gate Electrode>>

Finally, an Al film was formed on the gate insulating layer 15 through asputtering method so as to have a thickness of 100 nm. On the formed Alfilm, resist patterns were formed through photolithography and theresultant was subjected to etching to form a gate electrode 16 having apredetermined shape. Subsequently, the film was subjected to anannealing treatment using an oven for 1 hour at 300 degrees Celsius inthe air. This annealing treatment is generally performed in order toimprove transistor characteristics by decreasing interface defectdensity between the active layer and the gate insulating layer.

As described above, a top gate/top contact field-effect transistorpresented in FIG. 6 was completed.

Examples 11 to 12

<Production of Field-Effect Transistor>

In Examples 11 to 12, a top gate/bottom contact field-effect transistorwas produced in the same manner as in Example 10 except that the—Formation of source electrode precursor and drain electrodeprecursor—in Example 10 was changed to the below-described method.

The same evaluations as described in Example 1 were performed. Resultsare presented in Table 3.

—Formation of Source Electrode Precursor and Drain Electrode Precursor—

A metal layer 2 and a metal layer 3 were formed in the same manner as inthe production steps of the field-effect transistor in Example 10 exceptthat the target for forming the metal layer was changed to each targetdescribed in Table 3.

Examples 13 to 14

<Production of Field-Effect Transistor>

In Examples 13 to 14, a top gate/bottom contact field-effect transistorwas produced in the same manner as in Example 10 except that the—Formation of source electrode precursor and drain electrodeprecursor—and the <<Formation of active layer>> in Example 10 werechanged to the below-described methods.

The same evaluations as described in Example 1 were performed. Resultsare presented in Table 3.

—Formation of Source Electrode Precursor and Drain Electrode Precursor—

A metal layer 2 and a metal layer 3 were formed in the same manner as inthe production steps of the field-effect transistor in Example 10 exceptthat the target used for forming the metal layer was changed to eachtarget presented in Table 3.

<<Formation of Active Layer>>

A film of an active layer was formed in the same manner as in theproduction steps of the field-effect transistor in Example 10 exceptthat the sintered body target for forming the active layer was changedto each target described in the following Table 3.

Comparative Example 1

<Production of Field-Effect Transistor>

A top gate/bottom contact field-effect transistor presented in FIG. 20was produced in the same manner as in Example 1 except that theoxidation treatment of the source electrode and the drain electrode inExample 1 was omitted. In FIG. 20, the numerical reference 112 is asource electrode and the numerical reference 113 is a drain electrode.

The same evaluations as described in Example 1 were performed. Resultsare presented in Table 1.

Here, as a result of evaluation of transistor performances, highelectric current flowed in a state that gate voltage was not applied andswitching did not arise. Therefore, the calculated on/off ratio was 10¹.

Comparative Example 2

<Production of Field-Effect Transistor>

A top gate/top contact field-effect transistor presented in FIG. 21 wasproduced in the same manner as in Example 7 except that the stackingtreatment of the source electrode and the drain electrode in Example 7was omitted.

The same evaluations as described in Example 1 were performed. Resultsare presented in Table 2.

Here, as a result of evaluation of transistor performances, highelectric current flowed in a state that gate voltage was not applied andswitching did not arise. Therefore, the calculated on/off ratio was 10¹.

Comparative Example 3

<Production of Field-Effect Transistor>

A top gate/top contact field-effect transistor presented in FIG. 6 wasproduced in the same manner as in Example 10 except that the <<Formationof active layer>> was changed as described below and the oxidationtreatment was omitted.

The same evaluations as described in Example 1 were performed. Resultsare presented in Table 3.

Here, as a result of evaluation of transistor performances, highelectric current flowed in a state that gate voltage was not applied andswitching did not arise. Therefore, the calculated on/off ratio was 10¹.

<<Formation of Active Layer>>

A film of W-doped MgIn₂O₄ was formed on the substrate through a RFmagnetron sputtering method so as to have a film thickness of 50 nm.When the active layer was formed through sputtering, a polycrystallinesintered body having a constitution of MgIn_(1.99)W_(0.01)O₄ was used asa target. An argon gas and an oxygen gas were introduced as sputteringgasses. A total pressure was fixed to 1.1 Pa and oxygen concentrationwas set to 10% by volume. On the formed W-doped MgIn₂O₄, resist patternswere formed through photolithography and the resultant was subjected toetching to form an active layer 14 having a predetermined shape. In thethus-obtained active layer, In in MgIn₂O₄ underwent substitutionaldoping with W in a concentration of 0.5 mol %.

TABLE 1 Treatment Configuration of method of Surface work sourceelectrode source function Oxygen gas and electrode of source Target forconcentration drain electrode and electrode and forming in formationTransistor First Second drain drain electrode active of activecharacteristics layer layer electrode [eV] layer layer (on/off ratio)Example 1 Mo Al Heat 5.4

2.5% by

treatment volume Example 2 V Al Plasma 5.4

2.5% by

treatment volume Example 3 Ti Al Heat 5.5

10% by

treatment volume Example 4 Ti—Ta Al Heat 5.4

10% by

treatment volume Example 5 Ti—W Al Plasma 5.4

10% by

treatment volume Example 6 V—W Al Plasma 5.2

10% by

treatment volume Comparative Mo Al No 4.6

10% by

Example 1 treatment volume Alloy target (Ti—Ta) of Example 4: (Ti:Ta =98:2 (atomic ratio)) Alloy target (Ti—W) of Example 5: (Ti:W = 98:2(atomic ratio)) Alloy target (V—W) of Example 6: (V:W = 98:2 (atomicratio))

indicates data missing or illegible when filed

TABLE 2 Configuration Treatment Oxygen gas of source method of Targetfor concentration electrode and source electrode forming in formationTransistor drain electrode and active of active characteristics (metalregion) drain electrode layer layer (on/off ratio) Example 7 Ti—NbStacking treatment

10% by

(reactive sputtering volume using the same alloy target) Example 8 V—WStacking treatment

10% by

(reactive sputtering volume using the same alloy target) Example 9 Ti—NbStacking treatment

10% by

(oxide target + volume alloy target) Comparative Ti—Nb No

10% by

Example 2 treatment volume

indicates data missing or illegible when filed

TABLE 3 Treatment Surface work Configuration method function of Oxygengas of source of source source electrode Target for concentrationelectrode and electrode and forming in formation Transistor drainelectrode and drain drain electrode active of active characteristics(metal region) electrode [eV] layer layer (on/off ratio) Example 10 MoHeat 5.4

50% by

treatment volume Example 11 Ti Heat 5.4

50% by

treatment volume Example 12 Ti—Nb Heat 5.6

50% by

treatment volume Example 13 V Heat 5.4

50% by

treatment volume Example 14 Ti—V Heat 5.3

50% by

treatment volume Comparative Mo No 4.6

10% by

Example 3 treatment volume Alloy target (Ti—Nb) of Example 12: (Ti:Nb =98:2 (atomic ratio)) Alloy target (Ti—V) of Example 14: (Ti:V = 98:2(atomic ratio))

indicates data missing or illegible when filed

From Table 1, as comparing the work functions of Examples 1 to 6 withthe work function of Comparative Example 1, it is found that the workfunction values of Examples 1 to 6 were large and the surface-oxidizedlayers were formed on the surfaces of the source electrode and the drainelectrode. At this time, in Examples 1 to 3, a concentration of oxygengas, which was introduced when the film formation of the active layerwas performed, is optimized to form the oxygen regions on the surfacesof the source electrode and the drain electrode. As a result, it ispossible to obtain high transistor characteristics (on/off ratio) withthe carrier concentration being maintained. Meanwhile, in ComparativeExample 1, there is no oxidized layer beforehand on the surfaces of thesource electrode and the drain electrode, which results in oxidation ofthe electrodes at the interface between the active layer and the sourceelectrode and the interface between the active layer and the drainelectrode. As a result, high electric current flows even when the gatevoltage is not applied, resulting in a normally-on field-effecttransistor. The reason for this is as follows. Specifically, oxygenexcessively moved outside the active layer (i.e., the oxidesemiconductor was reduced). As a result, a carrier attributed to thegenerated oxygen vacancies induced increasing of the carrierconcentration of the oxide semiconductor, which was larger than thecarrier concentration optimized under the conditions of the filmformation, resulting in the active layer having low resistance.

The field-effect transistors of Examples 4 to 6 have a value of theon/off ratio higher by one digit than in Examples 1 to 3. The reason forthis is as follows. That is, the oxides formed on the source electrodeand the drain electrode underwent substitutional doping to increaseelectric conductivity (i.e., decrease in resistivity). As a result,contact resistivity was decreased at the interface between the sourceelectrode and the active layer and the interface between the drainelectrode and the active layer. Accordingly, when the oxide undergoingthe substitutional doping is formed between the source electrode and theactive layer and between the drain electrode and the active layer, morefavorable contact can be obtained.

From Table 2, comparison between Examples 7 to 9 and Comparative Example2 indicates that the same tendency can be obtained. This is because theoxide region formed through the stacking treatment served as a reductionpreventing layer for the active layer, which inhibited an amount of theoxygen vacancies generated in the active layer.

From Table 3, comparison between Examples 10 to 14 and ComparativeExample 3 indicates that the same tendency can be obtained. The reasonfor this is as follows. By excessively supplying oxygen gasconcentration introduced upon performing film formation of the activelayer, even when the oxide regions were formed on the source electrodeand the drain electrode through the heating in the oxidation treatment,the oxygen vacancies were not a dominant factor of the carriergeneration mechanism in the active layer, which could preventdeterioration of the characteristics. Here, even in a state that theoxygen vacancies are inhibited, the active layer functioned to exhibitswitching operation. This is because carriers are generated by thesubstitutional doping. In Comparative Example 3, a concentration ofoxygen gas introduced upon performing film formation of the active layerwas not excessive. Therefore, when oxidation of the electrodes arose atthe interface at the interface between the source electrode and theactive layer and the interface between the drain electrode and theactive layer, the oxide semiconductor of the active layer was reduced todecrease the resistivity and flow high electric current in a state thatgate voltage was not applied. As described above, when there is no oxideregion between the source electrode and the active layer and between thedrain electrode and the active layer beforehand, the electriccharacteristics of the active layer will be affected. Therefore, inorder to form the oxide regions as described in Examples 10 to 14, it isnecessary to change the conditions for forming the active layer to formthe oxide regions.

Aspects of the present disclosure are as follows.

<1> A field-effect transistor including:

a gate electrode, which is configured to apply gate voltage;

a source electrode and a drain electrode, which are configured to takeelectric current out;

an active layer, which is disposed between the source electrode and thedrain electrode and is formed of an oxide semiconductor; and

a gate insulating layer, which is disposed between the gate electrodeand the active layer,

wherein the source electrode and the drain electrode each include ametal region formed of a metal and an oxide region formed of one or moremetal oxides, and

wherein a part of the oxide region in each of the source electrode andthe drain electrode is in contact with the active layer, and rest of theoxide region is in contact with one or more components other than theactive layer.

<2> The field-effect transistor according to <1>,

wherein oxygen concentration in a region of the oxide region in each ofthe source electrode and the drain electrode decreases toward the metalregion, the region of the oxide region in each of the source electrodeand the drain electrode being in contact with the active layer.

<3> The field-effect transistor according to <1> or <2>,

wherein the metal is a simple substance of a transition metal or analloy thereof.

<4> The field-effect transistor according to any one of <1> to <3>,wherein the metal includes at least one selected from the groupconsisting of Ti, V, Nb, Ta, Mo, and W.

<5> The field-effect transistor according to any one of <1> to <4>,wherein the source electrode and the drain electrode each have a stackedstructure of a first layer and a second layer, the first layer includingthe metal region and the oxide region and the second layer being formedof a metal.

<6> The field-effect transistor according to <1> or <2>,

wherein the oxide includes a transition metal having a positive valenceand a substitutional dopant having a positive valence larger than thepositive valence of the transition metal, and

wherein the metal includes an element of the transition metal and anelement serving as a dopant with respect to the oxides.

<7> The field-effect transistor according to <6>,

wherein the element of the transition metal includes at least oneselected from the group consisting of Ti, V, Nb, Ta, Mo, and W.

<8> The field-effect transistor according to <6> or <7>,

wherein the element serving as a dopant with respect to the oxidesincludes at least one selected from the group consisting of V, Nb, Ta,Cr, Mo, W, Mn, and Re.

<9> The field-effect transistor according to any one of <1> to <8>,wherein the oxide semiconductor includes at least one selected from thegroup consisting of In, Zn, Sn, and Ti.

<10> The Field-Effect Transistor According to <9>,

wherein the oxide semiconductor includes at least one of alkaline earthelements.

<11> The field-effect transistor according to <9>,

wherein the oxide semiconductor includes at least one of rare earthelements.

<12> The field-effect transistor according to <9>,

wherein the oxide semiconductor is an n-type oxide semiconductor andundergoes a substitutional doping with at least one dopant selected fromthe group consisting of a bivalent cation, a trivalent cation, atetravalent cation, a pentavalent cation, a hexavalent cation, aheptavalent cation, and an octavalent cation, and

wherein a valence of the dopant is larger than a valence of a metal ionconstituting the oxide semiconductor, provided that the dopant isexcluded from the metal ion.

<13> A method for producing the field-effect transistor according to anyone of <1> to <12>, the method including

forming the source electrode, the drain electrode, and the active layerso that a part of the oxide region in each of the source electrode andthe drain electrode is in contact with the active layer.

<14> The method for producing the field-effect transistor according to<13>,

wherein the forming the source electrode, the drain electrode, and theactive layer includes:

a treatment of oxidizing a surface of a metal layer to form the sourceelectrode and the drain electrode each having the metal region and theoxide region; and

a treatment of forming the active layer so as to be in contact with thepart of the oxide region in each of the source electrode formed and thedrain electrode formed.

<15> The method for producing the field-effect transistor according to<13>,

wherein the forming the source electrode, the drain electrode, and theactive layer includes:

a treatment of forming the active layer; and

a treatment of forming the oxide region so that the part of the oxideregion is in contact with the active layer formed and further formingthe metal region on the oxide region to form the source electrode andthe drain electrode each including the oxide region and the metalregion.

<16> The method for producing the field-effect transistor according to<13>,

wherein the forming the source electrode, the drain electrode, and theactive layer includes:

a treatment of forming the active layer;

a treatment of forming a metal layer so that a part of the metal layeris in contact with the active layer formed; and

a treatment of oxidizing a surface of the metal layer to form the sourceelectrode and the drain electrode each including the metal region andthe oxide region so that the part of the oxide region is in contact withthe active layer formed.

<17> The method for producing the field-effect transistor according toany one of <13> to <16>,

wherein oxygen concentration in a region of the oxide region in each ofthe source electrode and the drain electrode decreases toward the metalregion, the region of the oxide region in each of the source electrodeand the drain electrode being in contact with the active layer.

<18> The method for producing the field-effect transistor according toany one of <13> to <17>,

wherein the metal is a simple substance of a transition metal or analloy thereof.

<19> The method for producing the field-effect transistor according toany one of <13> to <16>,

wherein the oxide includes a transition metal having a positive valenceand a substitutional dopant having a positive valence larger than thepositive valence of the transition metal, and

wherein the metal includes an element of the transition metal and anelement serving as a dopant with respect to the oxides.

<20> A display element including:

an optical control element configured to control light output accordingto a driving signal; and

a driving circuit including the field-effect transistor according to anyone of <1> to <12> and configured to drive the optical control element.

<21> The display element according to <20>,

wherein the optical control element includes an organicelectroluminescent element, an electrochromic element, a liquid crystalelement, an electrophoretic element, or an electrowetting element.

<22> An image display device configured to display an imagecorresponding to image data, the image display device including:

a plurality of display elements arranged in a form of matrix, each ofthe plurality of display elements being the display element according to<20> or <21>;

a plurality of wires configured to individually apply gate voltage andsignal voltage to the field-effect transistors in the plurality ofdisplay elements; and

a display control device configured to individually control the gatevoltage and the signal voltage of the field-effect transistors via theplurality of wires correspondingly to the image data.

<23> A system including:

the image display device according to <22>; and

an image-data-generating device configured to generate image data basedon image information to be displayed and to output the image data to theimage display device.

REFERENCE SIGNS LIST

2 metal layer

3 metal layer

10 field-effect transistor

11 substrate

12 source electrode

12A metal region

12B oxide region

13 drain electrode

13A metal region

13B oxide region

14 active layer

15 gate insulating layer

16 gate electrode

17 interlayer insulating layer

18 data lines

20 field-effect transistor

22 active layer

23 source electrode

24 drain electrode

25 gate insulating layer

26 gate electrode

40 field-effect transistor

302, 302′ display element

310 display

320, 320′ drive circuit

370 liquid crystal element

400 display control device

1. A field-effect transistor comprising: a source electrode and a drainelectrode; and an active layer formed of an oxide semiconductor, whereinthe source electrode and the drain electrode each include a metal regionformed of a metal and an oxide region formed of one or more metaloxides, and wherein a part of the oxide region in each of the sourceelectrode and the drain electrode is in contact with the active layer,and rest of the oxide region is in contact with one or more componentsother than the active layer.
 2. The field-effect transistor according toclaim 1, wherein oxygen concentration in a region of the oxide region ineach of the source electrode and the drain electrode decreases towardthe metal region, the region of the oxide region in each of the sourceelectrode and the drain electrode being in contact with the activelayer.
 3. The field-effect transistor according to claim 1, wherein themetal is a simple substance of a transition metal or an alloy thereof 4.The field-effect transistor according to claim 1, wherein the metalincludes at least one selected from the group consisting of Ti, V, Nb,Ta, Mo, and W.
 5. The field-effect transistor according to claim 1,wherein the source electrode and the drain electrode each have a stackedstructure of a first layer and a second layer, the first layer includingthe metal region and the oxide region and the second layer being formedof a metal.
 6. The field-effect transistor according to claim 1, whereinthe oxide includes a transition metal having a positive valence and asubstitutional dopant having a positive valence larger than the positivevalence of the transition metal, and wherein the metal includes anelement of the transition metal and an element serving as a dopant withrespect to the oxides.
 7. The field-effect transistor according to claim6, wherein the element of the transition metal includes at least oneselected from the group consisting of Ti, V, Nb, Ta, Mo, and W.
 8. Thefield-effect transistor according to claim 6, wherein the elementserving as a dopant with respect to the oxides includes at least oneselected from the group consisting of V, Nb, Ta, Cr, Mo, W, Mn, and Re.9. The field-effect transistor according to claim 1, wherein the oxidesemiconductor includes at least one selected from the group consistingof In, Zn, Sn, and Ti.
 10. The field-effect transistor according toclaim 9, wherein the oxide semiconductor includes at least one ofalkaline earth elements.
 11. The field-effect transistor according toclaim 9, wherein the oxide semiconductor includes at least one of rareearth elements.
 12. The field-effect transistor according to claim 9,wherein the oxide semiconductor is an n-type oxide semiconductor andundergoes a substitutional doping with at least one dopant selected fromthe group consisting of a bivalent cation, a trivalent cation, atetravalent cation, a pentavalent cation, a hexavalent cation, aheptavalent cation, and an octavalent cation, and wherein a valence ofthe dopant is larger than a valence of a metal ion constituting theoxide semiconductor, provided that the dopant is excluded from the metalion.
 13. A method for producing the field-effect transistor according toclaim 1, the method comprising forming the source electrode, the drainelectrode, and the active layer so that a part of the oxide region ineach of the source electrode and the drain electrode is in contact withthe active layer.
 14. The method for producing the field-effecttransistor according to claim 13, wherein the forming the sourceelectrode, the drain electrode, and the active layer includes: atreatment of oxidizing a surface of a metal layer to form the sourceelectrode and the drain electrode each having the metal region and theoxide region; and a treatment of forming the active layer so as to be incontact with the part of the oxide region in each of the sourceelectrode formed and the drain electrode formed.
 15. The method forproducing the field-effect transistor according to claim 13, wherein theforming the source electrode, the drain electrode, and the active layerincludes: a treatment of forming the active layer; and a treatment offorming the oxide region so that the part of the oxide region is incontact with the active layer formed and further forming the metalregion on the oxide region to form the source electrode and the drainelectrode each including the oxide region and the metal region.
 16. Themethod for producing the field-effect transistor according to claim 13,wherein the forming the source electrode, the drain electrode, and theactive layer includes: a treatment of forming the active layer; atreatment of forming a metal layer so that a part of the metal layer isin contact with the active layer formed; and a treatment of oxidizing asurface of the metal layer to form the source electrode and the drainelectrode each including the metal region and the oxide region so thatthe part of the oxide region is in contact with the active layer formed.17-19. (canceled)
 20. A display element comprising: an optical controlelement configured to control light output according to a drivingsignal; and a driving circuit including the field-effect transistoraccording to claim 1 and configured to drive the optical controlelement.
 21. The display element according to claim 20, wherein theoptical control element includes an organic electroluminescent element,an electrochromic element, a liquid crystal element, an electrophoreticelement, or an electrowetting element.
 22. An image display deviceconfigured to display an image corresponding to image data, the imagedisplay device comprising: a plurality of display elements arranged in aform of matrix, each of the plurality of display elements being thedisplay element according to claim 20; a plurality of wires configuredto individually apply gate voltage and signal voltage to thefield-effect transistors in the plurality of display elements; and adisplay control device configured to individually control the gatevoltage and the signal voltage of the field-effect transistors via theplurality of wires correspondingly to the image data.
 23. A systemcomprising: the image display device according to claim 22; and animage-data-generating device configured to generate image data based onimage information to be displayed and to output the image data to theimage display device.